Castillo had been the Cubs' primary catcher for most of the past two seasons, but had just eight starts this year and was batting only .163 with two homers and five runs batted in over 24 games.

Mike Zunino and Jesus Sucre have been handling the catching chores for the Mariners this season.

The 28-year-old Castillo is a career .252 hitter with 29 homers and 110 RBI in 310 games over six seasons from 2010-15 with the Cubs. He batted .274 in 113 games in 2013 and slumped to .237 in 110 games last season, but his power numbers went up from eight homers in 2013 to 13 last year.

Medina was in his third season with the Mariners and had a record of 1-0 with one save and a 3.00 ERA in 12 games before being optioned to Triple-A Tacoma earlier this month. He will stay in the minors at Triple-A Iowa.

The 26-year-old right-hander has 140 strikeouts in 137 innings with a career mark of 10-9 and a 2.82 ERA in 141 relief appearances since breaking into the big leagues in 2013.